A fuel cell is a cell whereby a reaction energy of a gas as a feed material is converted directly to electric energy, and a hydrogen-oxygen fuel cell presents no substantial effect to the global environment since its reaction product is only water in principle. Especially, a polymer electrolyte fuel cell employing a polymer membrane as an electrolyte, can be operated at room temperature to provide a high power density, as a polymer electrolyte membrane having high ion conductivity has been developed, and thus is expected to be a prospective power source for mobile vehicles such as electric cars or for small cogeneration systems, along with an increasing social demand for an energy or global environmental problem in recent years.
In a polymer electrolyte fuel cell, a proton conductive ion exchange membrane is commonly employed as a polymer electrolyte, and an ion exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups, is particularly excellent in the basic properties. In the polymer electrolyte fuel cell, gas diffusion type electrode layers are disposed on both sides of the ion exchange membrane, and power generation is carried out by supplying a gas containing hydrogen as a fuel and a gas (such as air) containing oxygen as an oxidizing agent to the anode and the cathode, respectively.
In the reduction reaction of oxygen at the cathode of the polymer electrolyte fuel cell, the reaction proceeds via hydrogen peroxide (H2O2), and it is worried that the electrolyte membrane may be deteriorated by the hydrogen peroxide or peroxide radicals to be formed in the catalyst layer. Further, to the anode, oxygen molecules will come from the cathode through the membrane, and it is worried that hydrogen peroxide or peroxide radicals may be formed at the anode too. Especially when a hydrocarbon membrane is used as the polymer electrolyte membrane, it is poor in the stability against radicals, which used to be a serious problem in an operation for a long period of time.
For example, the first practical use of a polymer electrolyte fuel cell was when it was adopted as a power source for a Gemini space ship in U.S.A., and at that time, a membrane having a styrene/divinylbenzene polymer sulfonated, was used as an electrolyte membrane, but it had a problem in the durability over a long period of time. As a technique to overcome such problems, a method of having a compound with a phenolic hydroxyl group or a transition metal oxide capable of catalytically decomposing hydrogen peroxide incorporated to the polymer electrolyte membrane (see Patent Document 1) or a method of supporting catalytic metal particles in the polymer electrolyte membrane to decompose hydrogen peroxide (see Patent Document 2) is also known. However, such a technique is a technique of decomposing formed hydrogen peroxide, and is not one attempted to suppress decomposition of the ion exchange membrane itself. Accordingly, although at the initial stage, the effect for improvement was observed, there was a possibility that a serious problem would result in the durability over a long period of time. Further, there was a problem that the cost tended to be high.
As opposed to such a hydrocarbon type polymer, an ion exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups as a polymer remarkably excellent in the stability against radicals, has been known. In recent years, a polymer electrolyte fuel cell employing an ion exchange membrane made of such a perfluorocarbon polymer is expected as a power source for e.g. automobiles or housing markets, and a demand for its practical use is increasing, and its developments are accelerated. In such applications, its operation with particularly high efficiency is required. Accordingly, its operation at higher voltage is desired, and at the same time, cost reduction is desired. Further, from the viewpoint of the efficiency of the entire fuel cell system, an operation under low or no humidification is required in many cases.
However, it has been reported that even with a fuel cell employing an ion exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups, the stability is very high in operation under high humidification, but the voltage degradation is significant in operation under low or no humidification conditions (see Non-Patent Document 1). Namely, it is considered that, also in the case of the ion exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups, deterioration of the electrolyte membrane proceeds due to hydrogen peroxide or peroxide radicals in operation under low or no humidification.
Patent Document 1: JP-A-2001-118591
Patent Document 2: JP-A-6-103992
Non-Patent Document 1: Summary of debrief session for polymer electrolyte fuel cells research and development achievement in 2000 sponsored by New Energy and Industrial Technology Development Organization, page 56, lines 16 to 24